Graphene is a two-dimensional allotrope of carbon, in which a planar sheet of sp2 hybridised carbon atoms is arranged in a ‘honeycomb pattern’ of tessellated hexagons. Essentially graphene is a single layer of graphite. Graphene is a semi metal with high room temperature charge carrier mobility. It is stable in ambient conditions and its electronic properties can be controlled through application of an electric field as with traditional silicon transistors (K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, “Electric field Effect in Atomically Thin Carbon Films”Science, Vol. 306, No. 5696, pp. 666-669, 2004).
The advent of graphene and subsequent discovery of its multitude of superior properties, has led to the identification of many other two-dimensional crystals through both chemical modification of graphene and exfoliation of other layered compounds. Other two dimensional materials which have been isolated include NbSe2, bismuth strontium calcium copper oxide (BSCCO) and MoS2. These are also stable and can exhibit complementary electronic properties to graphene, such as being insulators, semiconductors or superconductors.
Graphene's properties are typically encumbered by its proximity to most substrates. Although it is possible to suspend graphene, this is technologically unfavourable due to the fragile nature of these devices. However, boron nitride (BN); a two-dimensional layered material which is a good insulator) provides a good substrate which has a much smaller effect on graphene's properties than previously reported materials. This is manifest in the increase in electron mobility and decrease in charge inhomogeneity of graphene. It has also become possible to achieve very clean and precise transfer of thin crystal flakes to the surfaces of one another and devices can be prepared which involve two electrically isolated graphene layers.
This new area of research and progress in precise transferring of the crystals whilst maintaining their quality has resulted in the emergence of a new class of materials: two-dimensional-crystal based heterostructures. More specifically, there is the possibility to create hybrid materials, by stacking combinations of two-dimensional crystals with differing properties. These structures are interesting from both a fundamental and an application based point of view. It has, for instance, been shown that a trilayer stack of graphene/boron nitride/graphene is operable as a tunnelling transistor. This means that the size of the barrier (BN) for electrons to flow between the two separate graphene layers can be varied by a gate electrode. These tunnelling devices are intrinsically fast and may be suitable for high frequency applications. The on/off ratio was enhanced by replacing the boron nitride layer with that of a material with a smaller band gap such as MoS2.
As well as allowing the operation of tunnelling transistors, this layering of sheets of graphene and boron nitride or molybdenum disulfide has permitted the observation of phenomena such as Coulomb drag. Coulomb drag is where the flow of electrons in one graphene layer of the graphene/BN/graphene heterostructures were observed to ‘drag’ along electrons in the other layer. These effects have previously been seen in GaAlAs heterostructures but in the case of graphene-based heterostructures an important feature is the ability to make the separation so small that electrons are closer to their counterparts in the other layer than within their own layer. There is also the possibility to tune between electrons and holes, which is not possible in conventional semiconductors.
Graphene can now be synthesised and transferred onto a substrate with roll-to-roll processing, enabling the possibility of industrial production of devices such as touch screens (S Bae, H. Kim, X. Xu, J.-S. Park. Y Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song. Y-J. Kim. K. S. Kim, B. Özyilmaz, J.-H. Ahn, S. ljima Roll-to-roll production of 30-inch graphene films for transparent electrodes; Nature Nanotechnology, 5, 574-578, 2010).
Graphene is also intrinsically very strong. It has been found to be one of the strongest materials ever measured. This means that graphene is inherently able to withstand large deformation forces. Combined with graphene's ability to elastically stretch up to 20% this makes it suitable for flexible electronic applications. This is likely to be particularly important in the field of display technologies and opens the way to flexible displays based on graphene. Such materials may find utility in a number of hand-held and small portable devices in which a flexible display would be advantageous.
Transition metal dichalcogenides (TMDCs) are a group of layered materials that have been found to exfoliate to monolayer by both mechanical and chemical methods. Many of these various materials—MoS2, WS2, TaS2 to name a few—are structurally similar but have an array of electronic properties ranging from semiconducting to metallic depending on their exact composition and thickness. Tungsten disulfide (WS2) has various applications including solid state lubrication and industrial surface protection.
It is an aim of certain embodiments of the present invention to provide a heterostructure containing quantum wells. A further aim is to provide a vertical heterostructure. In certain embodiments it is an aim to produce multiple quantum well (MQW) devices which are easier to fabricate and/or have improved properties relative to conventional quantum well (QW) structures. Another aim of the invention is to provide a novel single quantum well (SQW) heterostructure. It is also an aim of certain embodiments to provide a heterostructure having a high electroluminescent quantum efficiency relative to currently available materials. It is a further aim of certain embodiments of the invention to provide a vertical heterostructure capable of incorporation into an electronic device such as an LED. It is an aim to provide a device having a quantum efficiency which is sufficient to produce a viable LED device. A further aim of the invention is to prepare LEDs having a higher quantum efficiency and/or which are capable of emitting a broader range of visible radiation compared with current LEDs.
It is a further aim of embodiments of the invention to provide a device which converts a higher percentage of the input energy into visible or near infra red radiation than those of the prior art. In other words it is an aim to provide a cell or device which has a higher energy conversion efficiency than those of the prior art. Yet another aim is to provide a heterostructure or an electronic device incorporating such a heterostructure which is more robust and/or more flexible and/or which has a greater longevity than those of the prior art.
This invention generally relates to new applications of graphene. Specifically, the invention relates to new graphene heterostructures, applications of graphene heterostructures and methods of making graphene heterostructures.
It is an aim of the invention to provide methods of making graphene heterostructures which are more energy efficient than existing methods. The methods may be quicker than existing methods. They may generate less waste than existing methods.
It is an aim of this invention to provide methods which allow access to graphene heterostructures which it is not possible to make using existing methods.
It is an aim of this invention to provide methods which allow the efficient production of graphene heterostructures on a larger scale than existing methods.
A further aim of the invention is the provision of new graphene heterostructures.
These heterostructures may have similar properties to existing graphene heterostructures but be easier to produce. The new graphene heterostructures may have improved properties compared to known graphene heterostructures, or they may have improved properties compared to known non-graphene based materials. The graphene heterostructures may have new properties not previously observed in graphene heterostructures. In particular, the new heterostructures may have new combinations of properties not previously observed in a single material, whether that material is graphene based or not graphene based.
Another aim of the invention is to provide heterostructures for use in new photonics devices, such as LEDs. Further advantages and aims of the invention will be apparent from the following description.
Embodiments of the following invention may achieve at least one of the above aims.